In microchip fabrication, processing chambers are used in accomplishing various fabrication steps performed on a wafer. For example, among the numerous varieties of steps are the non-exhaustive examples of annealing, cleaning, chemical vapor deposition, etching, oxidation, and nitridation.
Chambers generally comprise a substantially planar and horizontal pedestal upon which the wafer rests during the processing within the chamber. The pedestal may also be known as a platen, stage or susceptor. A lift mechanism comprising pins which breach the pedestal from underneath provides a controlled disengagement from, and engagement of the wafer to, the pedestal surface. Wafers are transferred into and out of the chamber, and onto and off of the lift mechanism pins, by way of a robotic mechanism including a robot blade or fork at the end of a robotic arm.
Retention of the wafer to the pedestal while in the chamber is referred to as chucking. Chucking may be accomplished by vacuum or mechanical means. Alternatively, retention of the wafer to the pedestal while in the chamber may be accomplished electrostatically by an electrostatic chuck.
Wafer placement or positioning in the chamber is a critical process parameter. Wafer position affects many other process parameters and steps. For example, mispositioned wafers may be subject to much lower yields due to edge effects, for example, in a sputtering process. Wafer mispositioning in one chamber may be perpetuated in subsequent chambers where a series of chambers are loaded by a common robotic arm since a wafer that is transferred to a robot blade from a chamber in which the wafer is mispositioned is likely to be mispositioned with respect to the blade. Wafer backside helium flow is another process parameter that can be adversely affected by wafer mispositioning. Backside helium flow that is too high is known to cause over-etching. Transfer shift is a main cause of mispositioned wafers. An out of calibration robotic transfer can result in a mispositioned wafer. Recalibration of wafer transfer has conventionally required opening the chamber to check wafer position and performing transfer calibration. Such chamber violation requires performance of a variety of operations prior to releasing the apparatus for production use. Such operations include venting of the chamber, removal of the chamber lid, gas distribution plate and all process kits. A wet cleaning of the chamber, lid and gas distribution plate as well as other ancillary maintenance operations also need to be performed. This maintenance can take significant time and manpower resources. Eight to twelve hours of apparatus down time is common. It is, however, generally desirable to avoid such processes.